1. Field of the Invention
The present invention relates to a method of manufacturing a piezoelectric element and a method of manufacturing a liquid ejection head, and more particularly, to technology for manufacturing an oriented piezoelectric element deposited by means of a sputtering method, or the like.
2. Description of the Related Art
In general, an inkjet recording apparatus, which forms a desired image by ejecting ink droplets onto a recording medium from an inkjet head, is used widely as a generic image forming apparatus. In the inkjet recording apparatus, piezoelectric elements (piezoelectric actuators) are suitable for use as pressure application devices for causing ink droplets to be ejected from the inkjet head.
Improvements in the printing characteristics, and in particular, the increase in the resolution and the increase in the printing speed are demanded in inkjet heads. For this purpose, it has been sought to increase the resolution and raise the printing speed by using a multiple-nozzle head structure in which nozzles are formed very finely and are also arranged at high density. In order to achieve a high-density arrangement of nozzles, compactification of the piezoelectric elements is highly necessary.
In order to make the piezoelectric elements more compact in size, it is valuable to reduce the thickness of the piezoelectric elements. For example, Japanese Patent Application Publication No. 10-286953 discloses technology in which, a lead dielectric layer (piezoelectric film) having a thickness of 3 μm is formed in order to achieve thin film thickness in the piezoelectric elements.
In the piezoelectric elements used in an inkjet head, if upper electrodes are used as address electrodes (individual electrodes), lower electrodes are used as ground electrodes (a common electrode), and the lower electrodes are grounded while the upper electrodes are applied with a positive voltage to drive the piezoelectric elements, then it is beneficial in terms of the cost of the devices, such as the ease of wiring, switching ICs (integrated circuits), and the like, and hence this wiring structure is appropriate. Hence, in general, the piezoelectric elements are subjected to polarization processing in such a manner that the piezoelectric elements deform in a direction that causes the liquid to be ejected when the lower electrodes are grounded while the upper electrodes are applied with a positive voltage.
Nevertheless, in Japanese Patent Application Publication No. 10-286953, a piezoelectric film is deposited on the lower electrode by sputtering, and therefore the polarization direction of the piezoelectric film is the direction from the lower electrode toward the upper electrode. More specifically, if a piezoelectric element 102 shown in FIG. 7 is produced by sputtering to deposit a piezoelectric film 112 on a lower electrode 110, then the polarization direction upon the deposition of the piezoelectric film 112 is the direction from the lower electrode 110 toward an upper electrode 114 (the upward direction indicated with an arrow A in FIG. 7). Then, in order to drive the piezoelectric element 102 having this composition to deform in the direction to apply pressure through a diaphragm 104 to liquid in a pressure chamber 106 to eject the liquid from a nozzle 108 (to deform in the direction indicated with an arrow B in FIG. 7), it is necessary to apply an electric field to the piezoelectric film 112 in the same direction as the polarization direction of the piezoelectric film 112. It is possible to adopt a method for this electric field application in which the upper electrode 114 is set to ground while a positive voltage is applied to the lower electrode 110 from a drive source 115. However if the upper electrode 114 is used as the ground electrode and the lower electrode 110 is used as the address electrode, then the wiring structure becomes complicated, and hence this method is not desirable from a structural and a manufacturing point of view.
In a general piezoelectric element, whether an electric field (voltage) is applied from the upper electrode toward the lower electrode, or whether an electric field is applied from the lower electrode toward the upper electrode, regardless of the direction of the applied electric field, it is possible to obtain the same amount of displacement provided that the electric field of the same intensity (voltage) is applied. On the other hand, in a piezoelectric element composed of a piezoelectric film formed by sputtering, the piezoelectric film innately exhibits an orientation (i.e., polarization direction) that is set when the piezoelectric film is deposited, and there is a phenomenon of variation in the amount of displacement of the piezoelectric element depending on the direction of the electric field applied to the piezoelectric film.
As shown in FIG. 8, when the electric field of intensity in the range of 0 kV/mm to −6.0 kV/mm is applied to the piezoelectric film in the piezoelectric element 102, then there is a direct proportional relationship between the applied electric field intensity and the amount of displacement (nanometers) of the piezoelectric element 102 (the characteristics represented with a line 120); however, when the applied electric field intensity is in the range of 0 kV/mm to 6.0 kV/=m, then not only is this direct proportional relationship lost, but also a phenomenon of reversal of the direction of displacement appears (the characteristics represented with a line 122).
As a method for avoiding these problems, there is a method of manufacturing an inkjet head by fabricating a piezoelectric element structure by sequentially depositing an upper electrode (114 in FIG. 7), a piezoelectric film (112 in FIG. 7) and a lower electrode (110 in FIG. 7) by sputtering onto a monocrystalline substrate of silicon (Si), magnesium oxide (MgO), or the like, (a so-called “dummy substrate”), and further forming a thin film serving as a diaphragm (104 in FIG. 7) onto the lower electrode, whereupon the thus fabricated piezoelectric element structure is mechanically inverted and transferred (bonded) to a pressure chamber structure formed in a silicon substrate or a glass substrate (corresponding to the base plate having the pressure chamber 106 formed therein in FIG. 7).
Further alternative methods of avoiding the aforementioned problems include a method where the upper electrode is used as the ground electrode and the lower electrode is used as the address electrode to be applied with a positive voltage, and a method where the lower electrode is used as the ground electrode and the upper electrode is used as the address electrode to be applied with a negative voltage to apply an electric field to the piezoelectric film in the negative direction (an electric field in the direction indicated with an arrow C in FIG. 7).
However, in the method where the previously fabricated piezoelectric element structure is mechanically inverted and transferred to the pressure chamber structure, costs are high since the monocrystalline substrate is thrown away after use. Furthermore, since the transfer bonding method is used, it is necessary to register the piezoelectric element structure and the pressure chamber structure accurately in position, but it is in fact extremely difficult to achieve accurate positioning between the piezoelectric element structure and the pressure chamber structure. The positioning accuracy of the piezoelectric element structure and the pressure chamber structure affects the ejection characteristics, and in an inkjet head having a plurality of nozzles, it is extremely difficult to fabricate the head having uniform ejection characteristics in the plurality of nozzles.
In the method of using the lower electrode 110 as the address electrode, then as shown in FIG. 9, if the diaphragm is made of silicon, a leakage current 130 arises and electrical cross-talk occurs whereby displacement is produced even in the piezoelectric elements that are not intended to be applied with the drive signal, and consequently there is a problem in that ink droplets are ejected from the nozzles that are not intended to be driven.
In the method of using the upper electrode 114 as the address electrode and applying the negative voltage to the upper electrode 114 to apply the electric field in the negative direction, the costs relating to the drive IC and the power source device required to apply the negative voltage are higher (several times to several tens of times higher) than those required for the positive voltage.
To summarize the above-described problems relating to the oriented piezoelectric film (namely, the piezoelectric film deposited by sputtering), the method that mechanically inverts the piezoelectric element structure and bonds same to the pressure chamber structure involves the problem of positioning accuracy during bonding, and the method that uses the lower electrode 110 as the address electrode involves the problem of electrical cross-talk occurring as a result of leakage current. Furthermore, the method that uses the electric field in the negative direction as the applied electric field involves the problem of increased costs in relation to the IC, and so on (see FIG. 10).